(1) Field of the Invention
This invention relates to a process for producing .alpha.-olefin polymers, and more particularly, it relates to a process for producing .alpha.-olefin polymers having a high crystallinity and a good particle form, with a high yield, by employing a preactivated catalyst which is suitable for slurry polymerization, and particularly gas phase polymerization, and further, as modifications of gas phase polymerization, a combination of gas phase polymerization with slurry polymerization or bulk polymerization.
(2) Description of the Prior Art
It is well known that .alpha.-olefins are polymerized in the presence of so-called Ziegler-Natta catalysts comprising a transition metal compound of IV-VI Groups of the Periodic Table and an organometallic compound of I-III Groups of the Table, and also including those obtained by modifying the combinations of the above two compounds with an electron donor, etc.
As the .alpha.-olefin polymerizations in the presence of such Ziegler-Natta catalysts, the following have been well known:
slurry polymerization carried out in a solvent such as n-hexane (e.g. Japanese patent publication No. Sho 32-10596 (1957), etc.); bulk polymerization carried out in a liquefied monomer such as liquefied propylene (e.g. Japanese patent publications No. Sho 36-6686 (1961), No. Sho 38-14041 (1963), etc.); and gas phase polymerization carried out in a gaseous monomer such as gaseous propylene (e.g. Japanese patent publications No. Sho 39-14812 (1964), No. Sho 42-17487 (1967)). Further, processes of bulk polymerization followed by gas phase polymerization are also known (e.g. Japanese patent publication No. Sho 49-14862 (1974), Japanese patent application laid-open No. Sho 51-135987 (1976), etc.).
Among them, gas phase polymerization process has been advantageous in that the recovery and reuse of solvent as used in slurry polymerization process are unnecessary and the recovery and reuse of liquefied monomer such as liquefied propylene as in bulk polymerization process are also unnecessary; hence the cost for recovering solvent or monomer is low and it is possible to simplify the apparatus for producing .alpha.-olefin polymers.
However, gas phase polymerization process has been disadvantageous in that since the monomer inside the polymerization vessel is present in gas phase, the monomer concentration is lower than those of slurry polymerization process or bulk polymerization process which lowers the reaction rate; hence, in order to increase the polymer yield per unit weight of catalyst, it has been necessary to prolong the retention time and enlarge the reactor for that purpose, or in order to enhance the catalyst activity, it has been necessary to employ a modified organoaluminum compound, which results in reducing the stereoregularity of polymer.
Further, the gas phase polymerization process has had drawbacks in that the polymer particles are not uniform due to the unevenness of catalyst particles; during the polymerization, monomer oligomerization occurs and also cohesion of polymer particles occurs and further, removal of polymerization heat is so insufficient that cohesion and agglomeration of polymer particles are promoted, resulting in clogging in the port through which the polymer is discharged from the polymerization vessel as well as in the transporting line, and all these make a long term stabilized, continuous operation difficult. With respect to polyolefin product, too, the resulting polymer particles become rough, the dispersibility of stabilizer in the product is poor or the uniformity of quality of polyolefin product becomes also poor due to the dispersion of physical properties of individual polymer particles.
The present inventors have made studies for obtaining .alpha.-olefin polymers with a high yield, even in case of gas phase polymerization process wherein the monomer concentration is relatively low.
The object of the present invention is to provide a process for producing .alpha.-olefin polymers having a high crystallinity and a good particle form, with a superior yield, by employing a highly activated catalyst obtained by a preliminary activation (abbreviated hereinafter to "preactivation"), which catalyst, even when employed in gas phase polymerization, enables the polymerization to exhibit its advantages fully.